1. Field of the Invention
The present invention generally relates to devices employing magnetic flux return paths such as electric motors. More particularly, the invention relates to magnetic devices having at least one flux return strip pressed between pole plates to complete a magnetic flux path or circuit. In another aspect of the invention, use of the flux return strips eliminates the need for a metal housing of ferrous material to complete the flux return path. This facilitates flexible design of light-weight mounting systems or integral mounting of the magnetic devices to mating components in product assemblies.
2. Discussion of the Prior Art
Magnetic devices of varying designs, such as the many forms of electric motors, are well known and frequently used for linear or rotating actuation in a myriad of products. The motors commonly use a magnetic flux return path about their periphery that incorporates ferrous material.
Many motors, such as the design shown in U.S. Pat. No. 5,567,998, use a ferrous metal motor housing that contacts pole plates at their outer radial edges to complete the flux return path while additionally serving as a protective barrier against contamination by environmental elements. It has been suggested in U.S. Pat. No. 5,243,248, that an improved motor design may include a flux return path formed by utilizing discrete windings of magnetic material wire surrounding the field windings of the motor. Still other motors have been manually assembled as a stacked assembly of loose components with rings of ferrous material placed between pairs of pole plates. A spring element was included in such motors to compress the stacked assembly within an outer metal housing to ensure good contact between components.
Motors that utilize radial pole plates and a ferrous metal outer housing for the magnetic flux return path have several disadvantages. First, it is difficult to achieve consistent contact between the outer edges of the pole plates and the housing. The housing must receive the pole plates or be formed around them. However, air gaps between the pole plates and housing decrease the efficiency of the flux return path and consequently, the performance of the motor. Therefore, to minimize air gaps between the inner wall of the housing and the outer edges of the pole plates, the housing must be crimped or otherwise put in tension to squeeze the pole plates. Moreover, the ability to maintain consistent contact between the pole plates and housing is jeopardized in motors subjected to thermal cycling due to the expansion and contraction of the housing.
Second, a flux return path formed by a ferrous metal housing which encloses the entirety of a motor or by windings of magnetic wire around a motor""s periphery often significantly adds to the mass of the motor. Motors, in turn, are usually mounted to other components to form assembled products. Therefore, greater motor mass directly contributes undesirable added mass to the overall product.
Third, greater motor mass often has disadvantageous secondary effects. In general, a heavier motor requires a mounting system of greater strength, and consequently of greater mass and potentially of greater complexity. It is common for motors having a ferrous metal housing, such as that shown in U.S. Pat. No. 5,567,998, to have some form of a metal mounting flange to permit attachment of the motor to other components of an assembled product. The metal mounting flange must be formed into or be attached to the housing, and appropriate fasteners are required. Thus, the additional mass of a ferrous metal housing also may indirectly contribute undesirable added mass and complexity to the overall product.
For motors that may have used discrete windings of magnetic material wire, such windings may contribute not only significant undesirable mass to the product, but also may require the added complexity of further winding operations.
Motors that may have used a manually assembled stack of pole plates with flux return rings therebetween would require some additional means of holding the assembly in compression to promote contact between the flux carrying elements. This necessarily involves additional components, such as a spring, and requires some additional structure, even if it is the housing itself, to contain and hold the assembly in compression. Such additions result in undesirable added mass and complexity in the construction of the motor.
In light of the shortcomings and undesirable features commonly found in magnetic devices such as electric motor constructions, it is desirable to have a relatively simple construction having a highly efficient magnetic flux return path that has at least a pair of pole plates located a fixed distance from each other and that such device have at least one flux return strip pressed between each pair of pole plates so as to maintain consistent contact between the flux return strip and the pole plates. It further is desirable that the cross-section of such flux return strip include an arcuate portion. It also is desirable to eliminate the ferrous motor housing and to have the electrical connector for the motor be adapted for integral mounting of the motor to a product assembly. Alternatively, it is desirable to have the motor include a light-weight plastic motor housing. It would be further advantageous to have the plastic motor housing adapted for mounting to a product assembly. The present invention overcomes disadvantages of prior motors, while providing the above-mentioned desirable features.
The purpose and advantages of the invention will be set forth in and apparent from the description and drawings that follow, as well as will be learned by practice of the invention disclosed and claimed herein.
The present invention is generally embodied in an improved magnetic device. In accordance with one aspect of the invention, the magnetic device comprises a stator assembly having a magnetic flux return path comprising at least a pair of pole plates located a fixed distance apart from each other and at least one flux return strip pressed between each pair of spaced apart pole plates. The magnetic device further has a rotor assembly having permanent magnetic material and being disposed for rotation relative to the stator assembly.
In a further aspect of the invention, the stator assembly is constructed with the pole plates over-molded into the stator and each flux return strip is pressed between a pair of the spaced apart pole plates and formed around the stator assembly.
In accordance with another aspect of the invention, the cross-section of each flux return strip includes an arcuate portion. The width of the cross-section of each flux return strip is increased when the strip is formed around the stator assembly.
In a still further aspect of the invention, the magnetic device further comprises an electrical connector having integral mounting flanges and the stator and rotor assemblies extend substantially forward of the electrical connector.
In another aspect of the invention, the magnetic device in combination with a mating component further comprises an electrical connector having integral mounting flanges for mounting the stator and rotor assemblies within a bore of the mating component.
In yet another aspect of the invention, the magnetic device has a non-ferrous housing disposed substantially around the stator and rotor assemblies, and having a seal that engages the electrical connector and the housing.